Development process of electrostatic latent images generally comprises a step in which electrostatically charged fine toner particles are attracted to and adsorbed onto the surface of an electrostatic latent image-bearing support by electrostatic attractive force.
As practical dry developing process, cascade development, fur brush development, magnetic brush developement, impression development or powder cloud development is widely known and in each development process toner particles to be used are required to have good flowability. With toner having low flowability it would be difficult to carry out smooth development and the image thus obtained would be degraded in its image quality.
As means for giving toner particles improved flowability it has been known in the art that hydrophobic silica is added to a toner composition as disclosed in Japanese Patent Publication for Open to Public Inspection No. 47345/1973. By this it was also possible to obtain a toner image of improved image quality.
However, it has been found to be disadvantageous that black spots are likely to occur in the thus obtained image, especially, when copying is repeatedly carried out for a long time. This phenomenon is considered to take place in the following process.
In electrophotography, so-called cleaning process to remove remaining toner on an electrostatic image-bearing support is usually necessary after image transfer process, in which toner image formed on the electrostatic image-bearing support is transferred onto another recording medium such as paper sheet. In the cleaning process the use of a cleaning blade is widely known because of its high cleaning efficiency and reduced contamination effects upon the surroundings. However, when a toner to which the aforementioned fine particles of hydrophobic silica are added is used, there is a tendency that black spots occur due to spot-like residuals on the electrostatic image-bearing support, which cause reduction of the photoconductivity on such portions, after passing of the support through the cleaning blade.
As for electrostatic latent image-bearing support photoconductive lightsensitive materials such as zinc oxide, selenium, cadmium sulfide and organic photoconductive materials are known and due to its photo-sensitivity, copying duarability or non-polluting property selenium photoconductive materials are widely used. However, the selenium photoconductive material has such technical disadvantage that white streaking, which is a white line- or belt-shaped under-developed portion on a photoconductive plate along its moving direction when a black original is copied, is likely to occur due to crystallization of selenium in the repeated copying under high temperature.
According to the research of the present inventors this crystallization is often either accelerated or restrained in the presence of certain metallic ion.
Further, it is essential for the successful electrostatic development and transfer processes in the electrophotography and the like processes that the toner particles acquire sufficient amount of electrostatic charge during frictional charging process. With a toner having insufficient charges, especially in the development under conditions of high temperature with high humidity, reduction in the density of the transferred image due to insufficient transfer property of the toner image or due to reduced development property is often observed.